MEASURING THE RELATIVE STRENGTHS OF A SET OF PARTIALLY COHERENT ACOUSTIC SOURCES

The measurement of the relative contributions to the acoustic field ma de by a set of sources, which by necessity must operate simultaneously , is of importance in many areas of noise control technology. A partia l solution to this problem, proposed in the 1970s, was the Polar Corre lation Technique. This recognized, initially, the Fourier transform re lationship between cross-spectra, measured in the acoustic far field, and the distribution of source strength of a line array of sources, ty pical of an aero-engine, for example. In a second development (1981) a parametric method was developed. Essentially, the position of the con tributing sources was assumed to be known and a least squares error pr ocedure was employed to establish an optimum fit between the source st rength distribution and the cross-spectral data. A major restriction, however, was the necessary assumption of mutually incoherent sources. In the present work it is demonstrated that this restriction is unnece ssary, albeit at the expense of some extra data processing. Specifical ly, we show that by employing several reference microphones, as oppose d to one for incoherent sources, both the amplitude of and coherence b etween an array of sources may be determined. The potential capability of the method is established both analytically and through extensive numerical simulation. (C) 1997 Academic Press Limited.